Re-queue message on exception

Question

I'm looking for a solid way of re-queuing messages that couldn't be handled properly - at this time.

I've been looking at http://dotnetcodr.com/2014/06/16/rabbitmq-in-net-c-basic-error-handling-in-receiver/ and it seems that it's supported to requeue messages in the RabbitMQ API.

else //reject the message but push back to queue for later re-try
{
    Console.WriteLine("Rejecting message and putting it back to the queue: {0}", message);
    model.BasicReject(deliveryArguments.DeliveryTag, true);
}

However I'm using EasyNetQ. So wondering how I would do something similar here.

bus.Subscribe<MyMessage>("my_subscription_id", msg => {
    try
    {
        // do work... could be long running
    }
    catch ()
    {
        // something went wrong - requeue message
    }
});

Is this even a good approach? Not ACK the message could cause problems if do work exceeds the wait for ACK timeout by the RabbitMQ server.

Answer 1

So I came up with this solution. Which replaces the default error strategy by EasyNetQ.

public class DeadLetterStrategy : DefaultConsumerErrorStrategy
{
    public DeadLetterStrategy(IConnectionFactory connectionFactory, ISerializer serializer, IEasyNetQLogger logger, IConventions conventions, ITypeNameSerializer typeNameSerializer)
    : base(connectionFactory, serializer, logger, conventions, typeNameSerializer)
    {
    }

    public override AckStrategy HandleConsumerError(ConsumerExecutionContext context, Exception exception)
    {
        object deathHeaderObject;
        if (!context.Properties.Headers.TryGetValue("x-death", out deathHeaderObject))
            return AckStrategies.NackWithoutRequeue;

        var deathHeaders = deathHeaderObject as IList;

        if (deathHeaders == null)
            return AckStrategies.NackWithoutRequeue;

        var retries = 0;
        foreach (IDictionary header in deathHeaders)
        {
            var count = int.Parse(header["count"].ToString());
            retries += count;
        }

        if (retries < 3)
            return AckStrategies.NackWithoutRequeue;
        return base.HandleConsumerError(context, exception);
    }
}

You replace it like this:

RabbitHutch.CreateBus("host=localhost", serviceRegister => serviceRegister.Register<IConsumerErrorStrategy, DeadLetterStrategy>())

You have to use the AdvancedBus so you have to setup everything up manually.

using (var bus = RabbitHutch.CreateBus("host=localhost", serviceRegister => serviceRegister.Register<IConsumerErrorStrategy, DeadLetterStrategy>()))
{
    var deadExchange = bus.Advanced.ExchangeDeclare("exchange.text.dead", ExchangeType.Direct);
    var textExchange = bus.Advanced.ExchangeDeclare("exchange.text", ExchangeType.Direct);
    var queue = bus.Advanced.QueueDeclare("queue.text", deadLetterExchange: deadExchange.Name);
    bus.Advanced.Bind(deadExchange, queue, "");
    bus.Advanced.Bind(textExchange, queue, "");

    bus.Advanced.Consume<TextMessage>(queue, (message, info) => HandleTextMessage(message, info));
}

This will dead letter a failed message 3 times. After that it'll go to the default error queue provided by EasyNetQ for error handling. You can subscribe to that queue.

A message is dead lettered when an exception propagates out of your consumer method. So this would trigger a dead letter.

static void HandleTextMessage(IMessage<TextMessage> textMessage, MessageReceivedInfo info)
{
    throw new Exception("This is a test!");
}

Answer 2

to the best of my knowledge, there is no way to manually ack, nack or reject a message with EasyNetQ.

I see you have opened an issue ticket with the EasyNetQ team, regarding this... but no answer, yet.

FWIW, this is a very appropriate thing to do. All of the libraries that I use support this feature set (in NodeJS) and it is common. I'm surprised EasyNetQ doesn't support this.